Consumers use primary and rechargeable (secondary) batteries in portable electronic devices such as radios, compact disc players, cameras, cellular phones, electronic games, toys, pagers and computers devices. When the service run time of a primary battery is over, the battery is usually thrown away. The service run time of a typical primary battery generally only permits usage of between approximately 40 and 70% of the total battery storage capacity. Once that portion of the initial stored energy has been used, the battery generally cannot supply enough voltage to drive a typical electronic circuit. When the useful life of these batteries is spent, the consumers usually throw the batteries away even though the battery still contains between approximately 30 and 60% of its storage capacity. Thus, extending the service run time of a primary battery by allowing a safe deeper discharge will reduce waste by allowing the electronic devices to use more of the storage capacity of the battery before throwing it away.
The overall life of a rechargeable battery, however, is primarily dependent upon the number of and the efficiency of the charge cycles. Rechargeable batteries may be charged and reused after each discharge cycle. As with a primary battery, after a percentage of the battery storage capacity has been used, the battery typically cannot supply enough voltage to drive an electronic circuit. Thus, each discharge cycle of a rechargeable battery may be extended if a deeper discharge of the battery is provided. The level of discharge of a rechargeable battery, however, has an impact on the number of and the efficiency of future charges of the rechargeable battery. In general, as the depth of discharge of a rechargeable electrochemical cell increases, the number of charge cycles that a rechargeable electrochemical cell may undergo decreases. The optimal discharge characteristics of particular types of rechargeable electrochemical cells, however, vary widely. In a Nickel Cadmium ("NiCd") battery, for example, a deep discharge is preferred because the battery may otherwise develop a "memory" effect if the battery is charged without being appropriately depleted resulting in a decreased capacity available for future charges. Deep discharge of a lithium battery, however, may damage the electrochemical cells. The service run time of a rechargeable electrochemical cell may generally be extended better by efficiently controlling the discharge and charge cycles of the particular cell such that the total number of charge cycles may be maximized and the amount of energy recovered from each discharge cycle of the electrochemical cell is also optimized.
In addition, consumers constantly demand smaller and lighter portable electronic devices. One of the primary obstacles to making these devices smaller and lighter is the size and weight of the batteries required to power the devices. In fact, as the electronic circuits get faster and more complex, they typically require even more current than they did before, and, therefore, the demands on the batteries are even greater. Consumers, however, will not accept more powerful and miniaturized devices if the increased functionality and speed requires them to replace or recharge the batteries much more frequently. Thus, in order to build faster and more complex electronic devices without decreasing their useful life, the electronic devices need to use the batteries more efficiently and/or the batteries themselves need to provide greater utilization of stored energy.
Some more expensive electronic devices include a voltage regulator circuit such as a switching converter (e.g., a DC/DC converter) in the devices for converting and/or stabilizing the output voltage of the battery. In these devices, multiple single-cell batteries are generally connected in series, and the total voltage of these batteries is converted into a voltage required by the load circuit by a converter. A converter can extend the run time of the battery by stepping down the battery output voltage in the initial portion of the battery discharge where the battery would otherwise supply more voltage, and therefore more power, than the load circuit requires, and/or by stepping up the battery output voltage in the latter portion of the battery discharge where the battery would otherwise be exhausted because the output voltage is less than the load circuit requires.
The approach of having the converter in the electronic device, however, has several drawbacks. First, the converters are relatively expensive to place in the electronic devices because every device manufacturer has specific circuit designs that are made in a relatively limited quantity and, thus, have a higher individual cost. Second, battery suppliers have no control over the type of converter that will be used with a particular battery. Therefore, the converters are not optimized for the specific electrochemical properties of each type of electrochemical cell. Third, different types of electrochemical cells such as alkaline and lithium cells have different electrochemical properties and nominal voltages and, therefore, cannot be readily interchanged. Additionally, the converters take up valuable space in the electronic devices. Also, some electronic devices may use linear regulators instead of more efficient switching converters such as a DC/DC converter. In addition, electronic devices containing switching converters may create electromagnetic interference (EMI) that may adversely affect adjacent circuitry in the electronic device such as a radio frequency (RF) transmitter. By placing the converter in the battery, however, the source of the EMI can be placed farther away from other EMI sensitive electronics and/or could be shielded by a conductive container of the battery.
Another problem with present voltage converters is that they typically need multiple electrochemical cells, especially with respect to alkaline, zinc-carbon, nickel cadmium (NiCd) and silver oxide batteries, in order to provide enough voltage to drive the converter. In order to avoid this problem, present converters usually require multiple electrochemical cells connected in series to provide enough voltage to drive the converter, which may then step the voltage down to a level required by the electronic device. Thus, due to the converter's input voltage requirements, the electronic device must contain several electrochemical cells, even though the electronic device itself may only require a single cell to operate. This results in wasted space and weight and prevents further miniaturization of the electronic devices.
Thus, a need exists to optimally use the stored charge of a rechargeable battery and optimize the depth of discharge before charging the battery in order to maximize its service run time. By designing batteries to provide a greater utilization of their stored energy, electronic devices can also use smaller or fewer batteries in order to further miniaturize portable electronic devices.